This invention generally relates to seal structures for Surface Effect Ships (SES) and more particularly to a bag and planer type of seal structure which includes lightweight planer members for minimizing the effect of impact loads.
Surface Effect Ships of the type also referred to as Captured Air Bubble (CAB) vessels utilize a cushion of air contained in a plenum chamber beneath the vessel for supporting the vessel above the surface of the ground or water. In marine SES vessels the plenum chamber is commonly defined as a space between a pair of spaced, depending sidewalls with bow and stern seals extending athwartships between respective fore and aft end portions of the sidewalls. Particular performance criteria with seal structures for the SES vessels include: preventing excessive loss of pressurized air from the plenum chamber through and around the seals; providing a strong seal structure which is lightweight and flexible enough to exhibit rapid response characteristics to various wave conditions; and minimizing the effect of large drag and impact forces exerted on the seals by the waves. Although various types of seal structures, such as the planer, the bag and finger, and the bag and planer types of seal construction have been proposed to accommodate these criteria, additional design requirements for the new generation of large, high speed SES vessels have precluded the use of many seal designs in such vessels. For example, large SES vessels generally require large, lightweight and flexible seal structures having rapid response characteristics to accommodate rapid variations in the water surface at high speeds. Further, the seal structures must be durable enough to endure impact and dynamic seal weight loads imposed thereon under severe sea state conditions.
It was found, for example, that when many prior types of seal structures are scaled up to fit large SES vessels, such seal designs become unduly heavy, ponderous and unresponsive to various sea state conditions. For instance, although the planing seal disclosed in U.S. Pat. No. 3,532,180 granted to Ford and Wilson appears to be adaptable to small vessels, it has been difficult to scale the seal structure up to the size required for large SES vessels. The Ford seal structure includes a membrane and a plurality of flexible rods or spring steel strips incorporated therein for supporting the membrane. However, the rods or steel strips do not lend themselves to scaling up in size since they become heavier and stiffer as their dimensions are increased, which accordingly reduces their response and planing characteristics.
One example of a proposed seal structure for large SES vessels is disclosed in U.S. Pat. No. 4,137,987 granted to M. Plackett. The Plackett seal generally comprises a flexible bag structure secured between two spaced sidewalls and a planer seal portion of juxtaposed, elongated planer members or planers connected to the hull beneath the bag structure. The planers are maintained in operative position by a cooperative arrangement of support stays, which extend in front of the membrane and are connected to the fore end regions of both the planers and the hull, and the retracting and geometry straps, which extend from the hull and are connected to the aft end portions of the planers. In operative position the elongated planers are designed to act as planing surfaces for riding over the surface wave discontinuities and for isolating the wave impact forces acting on the planers from the hull section of the vessel. This is generally accomplished by supporting the aft or free end portions of the planers with a flexible arrangement of retracting and geometry straps that permit the planers to deflect in the fore and aft as well as in the up and down directions. A biasing or equilibrium means for causing the planers to return to a predetermined equilibrium position is provided by the bag structure which, upon inflation, exerts an outward pressure on the stay members. However, large scale seal structures utilizing the planers embodied in the Plackett patent have proved to be rather heavy and subject to severe slamming loads, due to their weight and stiffness characteristics, as the planers encounter the waves at high speeds. The heavy weight of the planers also tends to cause the seal structure to be highly stressed under severe operating conditions, such as "snap back". Snap back of the planers occurs after the lower portion of the seal is forced toward the hull by a wave which, after the wave passes the seal structure, causes a gap between the lower portion of the seal and the surface of the water. Under the influence of gravity and the cushion pressure, the planers accelerate toward the water until they are restrained or "snatched" by the flexible retracting and geometry support straps. Under such condition severe stresses are exerted on both the retracting and geometry support straps and the planers. Attempts to strengthen the Plackett planers with various reinforcing means tends to reduce the flexibility and responsive characteristics of the planers which accordingly induces larger impact and drag forces on the seal structure.